The present invention relates generally to an electromagnetic actuator and, in particular, to an engine valve control apparatus.
One form of known electromechanical actuators includes an armature that moves back and forth along a linear travel path between two electromagnet cores. The armature functions as an actuating member and is operated against the force of two springs positioned on opposite sides of the armature. In an unactuated state, the armature is positioned midway between the two cores by the opposing springs.
Electromagnetic actuators of the above-described type are used, for example, for operating cylinder valves of internal combustion engines. Each cylinder valve is actuated by the armature of the associated electromagnetic actuator. The armature which, by virtue of the forces of the return springs, assumes its position of rest between the two electromagnets, is alternatingly attracted by the one or the other electromagnet, and, accordingly, the cylinder valve is maintained in its closed or open position. If the valve is to be operated, for example, to be moved from the closed position to the open position, the holding current flowing through the electromagnet functioning as the closing magnet is interrupted. As a result, the holding force of the electromagnet falls below the spring force and the armature, accelerated by the spring force, begins to move. After the armature traverses its position of rest, the motion of the armature is braked by the spring force of the oppositely located return spring. To catch and hold the armature in the open position of the cylinder valve, current is applied to the other electromagnet, then functioning as an opening magnet.
To securely catch the armature, because of the inductive behavior of the coils of the electromagnets, either the current supply has to be applied very early to ensure that the current attains the required magnitude in time, or a steep current increase bas to be effected by means of a relatively high magnitude voltage. The latter alternative requires a second supply voltage of higher magnitude than a first supply voltage for holding. The additional structural of a second supply can be saved in principle by applying very early the current to the opening or catching electromagnet. Early application of current, however, is disadvantageous from the point of view of energy economy because the current in such a case builds up over a relatively long period of time during which large losses occur. Further, to maintain defined operational modes, in such an operation the current has to be applied at a time when no current flows through the opposite electromagnet. Such a proceeding is required, for example, if for starting from the position of rest by alternating excitation of the two electromagnets, the oscillation should be approximately at the natural resonance frequency of the spring/mass system.
The U.S. Pat. No. 5,682,127 describes such an actuator and a method of switching supply power to the coils of the electromagnets. The supply voltage is alternately applied to the coils to cause a supply current to flow alternately therethrough to effect a reciprocating motion of the armature. The induced voltage appearing across one of the coils upon removal of the supply voltage is utilized to apply an induced current to the other coil until the supply voltage applied to the other coil is greater than the induced voltage and is capable of maintaining an attained current flow through the other coil.
The U.S. Pat. No. 5,775,276 shows an electromagnetic valve driving apparatus that reduces the electromagnetic force when the valve body is close to the end of the stroke. A flywheel circuit and a variable resistor for increasing the resistance of the flywheel circuit are utilized to decrease the current flowing in the electromagnet coil.
The present invention concerns an apparatus for operating an electromagnetic actuator coil from a low voltage DC power supply. An amplifier/power supply has an input connected to a low voltage power supply, an output connected to an actuator coil, and a charging path connected between the input and the output and including a selectively switchable switch connected between the output and a circuit ground. Turning on the switch charges an inductance of the actuator coil with current flowing from the power supply along the charging path. The amplifier/power supply also has a discharging path including a capacitor connected between the input and a junction of the output and the switch whereby turning off the switch discharges the inductance of the coil into the capacitor along the discharging path. Alternately switching on and off the switch causes operation in a booster mode.
The amplifier/power supply includes another discharging path having another selectively switchable switch connected in series with the capacitor between the input and the output. After the capacitor is charged to a maximum valve, alternately switching of the switches causes operation in a holding mode. When both of the switches are turned on, the current flowing in the coil increases rapidly and when both of the switches are turned off, the current flowing in the coil decreases rapidly.